RFC 4796 Content Attribute February 20071. Introduction
The Session Description Protocol (SDP) [1] is a protocol that is
intended to describe multimedia sessions for the purposes of session
announcement, session invitation, and other forms of multimedia
session initiation. One of the most typical use cases of SDP is
where it is used with the Session Initiation Protocol (SIP) [5].
There are situations where one application receives several similar
media streams, which are described in an SDP session description.
The media streams can be similar in the sense that their content
cannot be distinguished just by examining their media description
lines (e.g., two video streams). The 'content' attribute is needed
so that the receiving application can treat each media stream
appropriately based on its content.
This specification defines the SDP 'content' media-level attribute,
which provides more information about the media stream than the 'm'
line in an SDP session description.
The main purpose of this specification is to allow applications to
take automated actions based on the 'content' attributes. However,
this specification does not define those actions. Consequently, two
implementations can behave completely differently when receiving the
same 'content' attribute.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in BCP 14, RFC 2119 [3], and indicate requirement levels
for compliant implementations.
3. Related Techniques
The 'label' attribute [10] enables a sender to attach a pointer to a
particular media stream. The namespace of the 'label' attribute
itself is unrestricted; so, in principle, it could also be used to
convey information about the content of a media stream. However, in
practice, this is not possible because of the need for backward
compatibility. Existing implementations of the 'label' attribute
already use values from that unrestricted namespace in an
application-specific way. So, it is not possible to reserve portions
of the 'label' attribute's namespace without possible conflict with
already used application-specific labels.
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RFC 4796 Content Attribute February 2007
It is possible to assign semantics to a media stream with an external
document that uses the 'label' attribute as a pointer. The downside
of this approach is that it requires an external document.
Therefore, this kind of mechanism is only applicable to special use
cases where such external documents are used (e.g., centralized
conferencing).
Yet another way to attach semantics to a media stream is to use the
'i' SDP attribute, defined in [1]. However, values of the 'i'
attribute are intended for human users and not for automata.
4. Motivation for the New Content Attribute
Currently, SDP does not provide any means for describing the content
of a media stream (e.g., speaker's image, slides, sign language) in a
form that the application can understand. Of course, the end user
can see the content of the media stream and read its title, but the
application cannot understand what the media stream contains.
The application that is receiving multiple similar (e.g., same type
and format) media streams needs, in some cases, to know what the
contents of those streams are. This kind of situation occurs, for
example, in cases where presentation slides, the speaker's image, and
sign language are transported as separate media streams. It would be
desirable that the receiving application could distinguish them in a
way that it could handle them automatically in an appropriate manner.
+--------------------------------------+
|+------------++----------------------+|
|| || ||
|| speaker's || ||
|| image || ||
|| || ||
|+------------+| presentation ||
|+------------+| slides ||
|| || ||
|| sign || ||
|| language || ||
|| || ||
|+------------++----------------------+|
+--------------------------------------+
Figure 1: Application's Screen
Figure 1 shows a screen of a typical communication application. The
'content' attribute makes it possible for the application to decide
where to show each media stream. From an end user's perspective, it
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RFC 4796 Content Attribute February 2007
is desirable that the user does not need to arrange each media stream
every time a new media session starts.
The 'content' attribute could also be used in more complex
situations. An example of such a situation is an application
controlling equipment in an auditorium. An auditorium can have many
different output channels for video (e.g., main screen and two
smaller screens) and audio (e.g., main speakers and headsets for the
participants). In this kind of environment, a lot of interaction
from the end user who operates the application would be required in
absence of cues from a controlling application. The 'content'
attribute would make it possible, for example, for an end user to
specify, only once, which output each media stream of a given session
should use. The application could automatically apply the same media
layout for subsequent sessions. So, the 'content' attribute can help
reduce the amount of required end-user interaction considerably.
5. The Content Attribute
This specification defines a new media-level value attribute,
'content'. Its formatting in SDP is described by the following ABNF
(Augmented Backus-Naur Form) [2]:
content-attribute = "a=content:" mediacnt-tag
mediacnt-tag = mediacnt *("," mediacnt)
mediacnt = "slides" / "speaker" / "sl" / "main"
/ "alt" / mediacnt-ext
mediacnt-ext = token
The 'content' attribute contains one or more tokens, which MAY be
attached to a media stream by a sending application. An application
MAY attach a 'content' attribute to any media stream it describes.
This document provides a set of pre-defined values for the 'content'
attribute. Other values can be defined in the future. The pre-
defined values are:
slides: the media stream includes presentation slides. The media
type can be, for example, a video stream or a number of instant
messages with pictures. Typical use cases for this are online
seminars and courses. This is similar to the 'presentation' role
in H.239 [12].
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RFC 4796 Content Attribute February 2007
speaker: the media stream contains the image of the speaker. The
media can be, for example, a video stream or a still image.
Typical use cases for this are online seminars and courses.
sl: the media stream contains sign language. A typical use case for
this is an audio stream that is translated into sign language,
which is sent over a video stream.
main: the media stream is taken from the main source. A typical use
case for this is a concert where the camera is shooting the
performer.
alt: the media stream is taken from the alternative source. A
typical use case for this is an event where the ambient sound is
separated from the main sound. The alternative audio stream could
be, for example, the sound of a jungle. Another example is the
video of a conference room, while the main stream carries the
video of the speaker. This is similar to the 'live' role in
H.239.
All these values can be used with any media type. We chose not to
restrict each value to a particular set of media types in order not
to prevent applications from using innovative combinations of a given
value with different media types.
The application can make decisions on how to handle a single media
stream based on both the media type and the value of the 'content'
attribute. If the application does not implement any special logic
for the handling of a given media type and 'content' value
combination, it applies the application's default handling for the
media type.
Note that the same 'content' attribute value can occur more than once
in a single session description.
6. The Content Attribute in the Offer/Answer Model
This specification does not define a means to discover whether the
peer endpoint understands the 'content' attribute because 'content'
values are just informative at the offer/answer model [8] level. The
fact that the peer endpoint does not understand the 'content'
attribute does not keep the media session from being established.
The only consequence is that end user interaction on the receiving
side may be required to direct the individual media streams
appropriately.
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RFC 4796 Content Attribute February 2007
The 'content' attribute describes the data that the application
generating the SDP session description intends to send over a
particular media stream. The 'content' values for both directions of
a media stream do not need to be the same. Therefore, an SDP answer
MAY contain 'content' attributes even if none were present in the
offer. Similarly, the answer MAY contain no 'content' attributes
even if they were present in the offer. Furthermore, the values of
'content' attributes do not need to match in an offer and an answer.
The 'content' attribute can also be used in scenarios where SDP is
used in a declarative style. For example, 'content' attributes can
be used in SDP session descriptors that are distributed with Session
Announcement Protocol (SAP) [9].
7. Examples
There are two examples in this section. The first example, shown
below, uses a single 'content' attribute value per media stream:
v=0
o=Alice 292742730 29277831 IN IP4 131.163.72.4
s=Second lecture from information technology
c=IN IP4 131.164.74.2
t=0 0
m=video 52886 RTP/AVP 31
a=rtpmap:31 H261/9000
a=content:slides
m=video 53334 RTP/AVP 31
a=rtpmap:31 H261/9000
a=content:speaker
m=video 54132 RTP/AVP 31
a=rtpmap:31 H261/9000
a=content:sl
The second example, below, is a case where there is more than one
'content' attribute value per media stream. The difference with the
previous example is that now the conferencing system might
automatically mix the video streams from the presenter and slides:
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RFC 4796 Content Attribute February 2007
v=0
o=Alice 292742730 29277831 IN IP4 131.163.72.4
s=Second lecture from information technology
c=IN IP4 131.164.74.2
t=0 0
m=video 52886 RTP/AVP 31
a=rtpmap:31 H261/9000
a=content:slides,speaker
m=video 54132 RTP/AVP 31
a=rtpmap:31 H261/9000
a=content:sl
8. Operation with SMIL
The values of 'content' attribute, defined in Section 5, can also be
used with Synchronized Multimedia Integration Language (SMIL) [11].
SMIL contains a 'param' element, which is used for describing the
content of a media flow. However, this 'param' element, like the
'content' attribute, provides an application-specific description of
the media content.
Details on how to use the values of the 'content' attribute with
SMIL's 'param' element are outside the scope of this specification.
9. Security Considerations
An attacker may attempt to add, modify, or remove 'content'
attributes from a session description. Depending on how an
implementation chooses to react to the presence or absence of a given
'content' attribute, this could result in an application behaving in
an undesirable way; therefore, it is strongly RECOMMENDED that
integrity protection be applied to the SDP session descriptions.
Integrity protection can be provided for a session description
carried in an SIP [5], e.g., by using S/MIME [6] or Transport Layer
Security (TLS) [7].
It is assumed that values of 'content' attribute do not contain data
that would be truly harmful if it is exposed to a possible attacker.
It must be noted that the initial set of values does not contain any
data that would require confidentiality protection. However, S/MIME
and TLS can be used to protect confidentiality, if needed.
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RFC 4796 Content Attribute February 200710. IANA Considerations
This document defines a new 'content' attribute for SDP. It also
defines an initial set of values for it. Some general information
regarding the 'content' attribute is presented in the following:
Contact name: Jani Hautakorpi <Jani.Hautakorpi@ericsson.com>.
Attribute name: 'content'.
Type of attribute Media level.
Subject to charset: No.
Purpose of attribute: The 'content' attribute gives information from
the content of the media stream to the receiving application.
Allowed attribute values: "slides", "speaker", "sl", "main", "alt",
and any other registered values.
The IANA created a subregistry for 'content' attribute values under
the Session Description Protocol (SDP) Parameters registry. The
initial values for the subregistry are as follows:
Value of 'content' attribute Reference Description
---------------------------- --------- -----------
slides RFC 4796 Presentation slides
speaker RFC 4796 Image from the speaker
sl RFC 4796 Sign language
main RFC 4796 Main media stream
alt RFC 4796 Alternative media stream
As per the terminology in RFC 2434 [4], the registration policy for
new values for the 'content' parameter shall be 'Specification
Required'.
If new values for 'content' attributes are specified in the future,
they should consist of a meta description of the contents of a media
stream. New values for 'content' attributes should not describe
things like what to do in order to handle a stream.
11. Acknowledgements
The authors would like to thank Arnoud van Wijk and Roni Even, who
provided valuable ideas for this document. We wish to also thank Tom
Taylor for his thorough review.
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RFC 4796 Content Attribute February 2007
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